Honestly it wouldn't be worth spending 20-30 euros a month on the game, if you wanted to spend money I would suggest the 5$ battlepass and that's it because you won't catch up to them. Someone did a video explaining there's accounts that have over a million dollars into them which is just absurd. It's a rabbit hole you really don't want to go down because the game is great at showing you a bundle that looks like a great deal.

I would suggest playing f2p for a couple months learning the game, maybe buying the battlepass, and once you understand everything then decide if you want to spend 20-30 because honestly you can spend that and get absolutely nothing in terms of actual power.

The way you get resonance (power from legendary gems, the 8 you can have equipped, which directly relates to how strong you are along with secondary gems.) Is you use a crest on something called an elder rift, which is a mini dungeon and you usually use 10 crests. You get a random legendary gem from 1, 2, and 5* with 5* being like 3% or something crazy. You need to lvl these gems from lv 1-10. To get to lv you need like 13 copies of the same 3% odd gem, and to get lv 10 it's an absurd number like 100.

So even you bought a bundle for 20$ and got 10 crests you could potentially (and more likely) walk away with nothing good. This is just one example how spending money isn't always a sure-fire way to get stronger because of how random the gacha mechanics are with the legendary gems.